ABSTRACT

This study investigates the prediction of residual stresses developed in shielded metal arc welding of ASTM A36 mild steel platesvia simulation and experiments. The specific objectives were to simulate the shielded manual metal arc welding process by using the finite element method in ANSYS Multiphysics Version 14, to produce experimental samples of butt welded ASTM A36 mild steel plates, to determine the residual stresses developed in the weldment of the steel plates and those generated from the Finite Element Model Simulation, and to establish correlation between experimental and predicted values of residual stress. Findings indicate that the maximum temperature was 1827°C while that at the end of the plate was maintained at around 27°C. From the Finite Element Model Simulation, the transverse residual stress in the x direction (σx) had a maximum value of 375MPa (tensile) and minimum value of -183MPa (compressive) while in the y direction (σy), the maximum value of 172MPa (tensile) and minimum value of 0.The longitudinal stress in the x direction (σx) indicated a maximum value of 355MPa (tensile) and a minimum value of -10MPa (compressive) while in the y direction (σy), the maximum value was 167MPa and the minimum value of the residual stress was -375MPa. The experimental values as measured by the X-Ray diffractometer were similar as transverse residual stress (σx) along the weld line in the transverse x directionvaried from 353MPa (tensile) to -209MPa (compressive) while in the y direction, stress (σy) along the weld line varied from 177MPa (tensile) to 0. The longitudinal stress measured by the X-Ray diffractometer in the x direction (σx) varied from 339MPa (tensile) to 0 (compressive) while in the y direction (σy) varied from 171MPa (tensile) to -366MPa (compressive). The result of the correlation coefficient test between the experimental and finite element results of residual stresses the was close to unity (1) which indicates a positive uphill linear relationship. The result of the F-Test conducted was also close to unity (1) which indicates the level of variance between the experimental and finite element results of residual stresses was not significant. Based on these results, it was established that using the 3D FEM analysis, results of residual stresses obtained was in good agreement with the experiment.

 

 

TABLE OF CONTENTS

Cover Page ……………………………………………………………………………………………. i
Title Page ……………………………………………………………………………… ii
Declaration …………………………………………………………………………. iii
Certification………………………………………………………………………… iv
Dedication .…………………………………………………………………………. v
Acknowledgements…………………………………………………………………. vi
Abstract ……………………………………………………………………………..………. viii
Table of Content …………………………………………………………..………. ix
List of Tables ………..………………………………………………………………… xv
List of Figures ………………………………………………………………….….. xvi
CHAPTER ONE: INTRODUCTION
1.0: Background to the Study …………………………………………………… 1
1.1: Statement of Problem ………………………………………………………. 3
1.2: Aim and Objectives ……..…………………………………………………… 3
1.2.1: Aim …………………………………………………………………………………………… 3
1.2.2: Objectives …………………………………………………………………………………… 3
1.6: Justification …………………………………………………………………………………. 4
1.7: Scope of Research ………………………………………………………………………… 4
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CHAPTER TWO: LITERATURE REVIEW
2.0: Introduction …………………………………………………………………………………. 5
2.1: Welding ………………………………………………………………………………………. 5
2.1.1: Types of Electric Arc Welding ……………………………………………………….. 5
2.1.1.1:Flux-Cored Arc Welding (FCAW) ………………………………………………….. 6
2.1.1.2:Gas Metal Arc Welding (GMAW) …………………………………………………… 6
2.1.1.3:Gas Tungsten Arc Welding (GTAW) ……………………………………………….. 6
2.1.1.4:Plasma Arc Welding (PAW) ………………………………………………………….. 6
2.1.1.5:Shielded Metal Arc Welding (SMAW) …………………………………………….. 7
2.1.1.6:Submerged Arc Welding (SAW)………………………………………………………. 7
2.1.2: Other Types Of Welding ……………………………………………………………….. 7
2.1.2.1:Oxy-fuel welding and cutting………………………………………………………….. 7
2.1.2.2:Resistance welding……………………………………………………………………….. 8
2.1.2.3:Spot welding ……………………………………………………………………………….. 8
2.1.2.4 Energy Beam ……………………………………………………………………………….. 8
2.2: Residual Stresses …………………….…………………………………………. 8
2.2.1: Causes of Residual Stress ……………..……………………………………… 9
2.2.2: Types of Residual Stress ………………..………..……………………..….. 9
2.2.3: Effects of Residual Stress ……………………………..…………………..…. 10
2.2.3.1:Distortion…………………………………………………………………………….…. 10
2.2.3.2:Crack Initiation and Propagation …………….……………………………… 10
2.2.3.3:Peen Formation (Controlled Distortion) …………………………………… 11
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2.2.3.4:Fretting …………………………………………………..………………………….. 11
2.2.3.5:Stress Corrosion Cracking (SCC) and Hydrogen Initiated Cracking ……… 11
2.3: Finite Element Analysis ………………………………………………………………… 12
2.3.1: Finite Element Analysis of Welding ………………………………………………… 12
2.3.2: Two-Dimensional vs. Three-Dimensional Modeling …………………………… 16
2.3.3: Finite Element Analysis Type …………………………………………………………. 18
2.3.3.1:Thermal Analysis …………………………………………………………………………. 19
2.3.3.2:Structural Analysis ……………………………………………………………………….. 25
2.4: Experimental Process ……………………………………………………………………. 27
2.4.1: Experimental Preparation of specimen ……………………………………………… 28
2.4.1.1:Shielded Metal Arc Welding ………………………………………………………….. 28
2.4.1.2:Filler Metal (Electrode) …………………………………………………………………. 30
2.4.1.3 Wire Brush ……………………………………………………………………………. ………. 32
2.4.1.4 Clamps ………………………………………………………………………………………. 32
2.4.1.5 Nitric Acid Solution………………………………………………………………………. 33
2.4.1.6 XMAS 2.0 Software …………………………………………………………………….. 33
2.4.1.7:Experimental Specimen …………………………………………………………………. 33
2.4.1.8:Weld Specimen Geometry …………………………………………………………….. 33
2.4.2: Experimental Measurement of Residual Stress …………………………………… 35
2.4.2.1:X-Ray Diffraction Technique ………………………………………………………… 35
2.4.2.2:Neutron diffraction Technique ………………………………………………………… 39
2.4.2.3:Ring-Core Technique ……………………………………………………………………. 40
2.4.2.4:Sectioning Technique …………………………………………………………………… 40
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2.4.2.5:Ultrasonic Method ……………………………………………………………………….. 41
2.4.2.6:Hole Drilling Method ……………………………………………………………………. 41
2.5: Review of Related Works ……………………………………………………………… 43
CHAPTER THREE: MATERIALS, EQUIPTMENT AND METHODS
3.0: Introduction …………………………………………………………………………………. 46
3.1: Materials…………………………………………….……………..……………. .. 46
3.2: Equipment ………………………………………………………………………………… 46
3.2.1: Shielded Manual Metal Arc Welding Machine ………………………………….. 47
3.2.2: X-Ray Diffractometer …………………………………………………………………… 47
3.2.3: Computer System ………………………………………………………………………… 48
3.3: Methods ……………………………………………………………………………………… 49
3.3.1: The Welding Process …………………………………………………………………… 49
3.3.2: Welding Precautions …………………………………………………………………… 5o
3.3.3: X-Ray Diffraction Measurement of Residual Stress …………………………… 51
3.3.4: Precautions in X-Ray diffraction Measurement of Residual Stress………… 52
3.4: Finite Element Model ………………………………………………………….. 52
3.4.1: Thermal Analysis ……………………………………………………………………….. 53
3.4.2: Structural Analysis …………………………………………………………………….. 55
3.5: Statistical Analysis of Results ……………………………………………………….. 57
CHAPTER FOUR: RESULTS AND DISCUSSION
4.0: Introduction ………………………………………………………………………………….. 58
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4.1: Results ……………………………………………………………….…………….…… 58
4.1.1: Temperature Distribution …………………………………………..………… 58
4.1.2: Stress Intensity Plot ………………………..……………………..…………… 59
4.1.3: Experimental Samples of Butt Welded ASTM A36 low Carbon Steel
produced …………………………………………………………………….……………. 60
4.1.4: Residual Stress Values Obtained ………………………………………………… 60
4.1.4.1:Experimental Residual Stress Values Obtained …………………………………. 60
4.1.4.2:Residual Stress Generated from Finite Element Simulation …………………. 64
4.1.5: Comparison of residual Stress from X-Ray Diffraction and Finite Element Model Simulation ……………………………………………………………………….. 66
4.1.6: Statistical Analysis of Result ………………………………………………………… 68
4.2: Discussion of Results …………………………………………………………………… 69
4.2.1: Temperature Distribution ……………………………………………………………… 69
4.2.2: Stress Intensity Plot …………………………………………………………………….. 70
4.2.3: Experimental Samples of Butt Welded ASTM A36 Low Carbon Steel Produced ……………………………………………………………………………………. 70
4.2.4: Residual Stress Values Obtained ……………………………………………………. 70
4.2.4.1:X-Ray Diffraction Experiment Values of Residual Stress Measured ………. 70
4.2.4.2:Finite Element Simulation Values of Residual Stress Generated …………… 71
4.2.5: Comparison of residual Stress from X-Ray Diffraction and Finite Element Model Simulation ………………………………………………………………………… 72
4.2.6: Statistical Analysis of Result …………………………………………………………. 73
4.2.6.1:Correlation Coefficient ……………………………………………………………….. 73
4.2.6.2:Ftest ………………………………………………………………………………………… 74
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CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS
5.0 Introduction ……………………………………………………………………………….. 76
5.1: Conclusion …………………………………………………………………. 76
5.2: Recommendations …………………………………………………………… 77
REFERENCES ………………………………………………………………………………………… 78

 

 

CHAPTER ONE

 

INTRODUCTION
1.0 Background of the study
The use of the Finite Element Method (FEM) in product development is now well establishedhowever, its use in manufacturing processes is not very common and is part of the field of new applications in computational mechanics. The most important reason for this development is the industrial need to improve productivity and quality of products and to have better understanding of the influence of different process parameters. The modelled phenomena play an important role at various stages of the production of steel parts, for example, welding, heat treatment and casting, among others.
The importance of these applications lies in determining the evolution of stresses and deformations to predict, for example, susceptibility to cracking and thus prevent failures during manufacturing or even service. Furthermore, this simulation tool can be used to optimize some aspects of the manufacturing process.Welding is defined by the American Welding Society (AWS) as a localized coalescence ofmetals or non-metals produced by either heating of the materials to a suitable temperature with or without the application of pressure, or by the application of pressure alone, with or without the use of filler metal (Mackerle, 2004). Welding techniques are one of the most important and most often used methods for joining pieces in industry. Any information about the shape, size and residual stress of a welded piece is of particular interest to improve quality.
The analysis of welding processes involves several branches of Physics, and requires thecoupling of different models addressed to describe the behaviour of a phenomenological system. Many of these models have been implemented numerically and are being used in an efficient way to solve the problems on an individual basis.
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The problem of welding distortion during fabrication processes causes to thedimensional inaccuracies and misalignments of structural members, which can result in corrective tasks or rework when tolerance limits are exceeded. This in turn, increases the production cost and leads to delays. In fabrication and design industries, expenses for rework such as straightening could cost lots of thousands. Therefore, the problems of distortion and residual stresses are always of great concern in welding industry. In order to deal with this problem, it is necessary to define prediction of the amount of distortion resulting from the welding operations. One way to predict the distortion and shrinkage of steel welding is through numerical analysis such as finite element analysis (FEA) (Xu, 2010).
Once the techniques of prediction of the distortion and shrinkage are identified, then theproblems can be controlled accordingly.Within the welding procedures, there are many factors such as welding process type,welding process parameters, welding sequence, preheat patterns, level of constraint and joint details that contribute to the distortion of the welded structure. Knowing which parameters have a major effect on the quality of the weld and which parameters give the most significant effects on the weld quality are the main issues in welding industry. The research activity in welding simulation started decades ago. Rosenthal (1946) was among the first researchers to develop an analytical solution of heat flow during welding based on conduction heat transfer for predicting the shape of the weld pool for two and three dimensional welds. Understanding of the theory of heat flow is essential in order to study the welding process analytically, numerically or experimentally since the pioneering work of Rosenthal (1946), considerable interest in the thermal aspects of welding was expressed by many researchers such as Andrea Capriccioli (2009), and Heinze et al (2012)
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1.1 Statement of the Problem
The existence of residual stresses in welded structures has caused fatigue and distortion of machine parts in service and were responsible for failure. These stresses if not properly controlled can lead to the loss of lives and property. Various trial and error welding runs have to be carried out while the welding parameters are varied until optimum conditions are determined. This is wasteful of time, material and finance. Thus the need to incorporate Finite Element Analysis prediction of residual stresses by computational methods to first determine satisfactory welding parameters before actual production is carried out.
1.2 Aim and Objectives
The aim and objectives of this research are briefly highlighted below:
1.2.1 Aim
The Aim of this study is to develop a Finite Element Model for predicting residual stresses in ShieldedMetal Arc Welding of ASTM A36 low Carbon Steel Plates.
1.2.2 Objectives
The Specific objectives of the study are outlined below:
i. To simulate the shielded manual metal arc welding process by usingthe finite element method in ANSYS Multiphysics Version 14 software.
ii. To produce experimental samples of butt welded ASTM A36 mild steel plates.
iii. To determine the residual stresses developed in the weldment of the steel plates and those generated from the Finite Element Model Simulation.
iv. To establish correlation between experimental and predicted values of residual stress.
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1.3 Justification
This research is intended to highlight the need for integration of computer aided methods in predicting residual stresses which cause failure during the service of manufactured products. Thus eliminate the waste of time, finance and also save lives. It also aims to minimize production and testing of multiple production setups in other to determine optimum production parameters.
1.4 Scope of Research
The scope of this research would cover ANSYS Multiphysics simulation of the welding process and the corresponding experimental X-Ray diffraction measurement of Residual stresses.
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